This invention relates to a single-line communications system wherein the status of remote components is monitored by return signals transmitted to the system head-end along the single transmission line.
In communications systems, such as a CATV system, wherein information signals are sent from a central point to a number of remote locations, it is important for system reliability to be able to periodically monitor the operational status of the different system components placed along the transmission path. Since communications systems of significant length require the introduction of power at various locations along their lengths to power components which compensate for attenuation along the line, the status of the power supplies and any standby equipment associated therewith must be continually monitored and evaluated so that prompt corrective action can be taken to provide continuous operation throughout the system. A cable television system employs a main cable originating at the head-end where the information signals are introduced with branch cables extending outwardly therefrom with intermediate amplifying devices distributed throughout the entire system. These distributed amplifiers are necessary to compensate for transmission lines losses and attenuation, plus any noise signals introduced from the power supplies so that the signal received by the furthest subscriber is substantially equivalent to that received by a subscriber proximate to the system head-end.
Initially, cable television systems required individual cable subscribers to report malfunctions at their localized reception point by means other than the system itself, such as by telephone. The location of the subscriber reporting a non-functioning display terminal which was closest to the head-end site was utilized to determine where to look for the fault. Repair personnel were dispatched to this point and began to track back through the system. The process was time consuming and the lengthy disruptions of programming to subscribers resulted in a reduction in subscriber renewals and made the enlisting of new subscribers difficult. An alternative approach to localizing faults that has been tried in the past relied on the transmission of a unique test signal through the cable system while maintenance personnel tapped into the cable at a variety of points to monitor the test signals. Faults were then localized between test points. When possible, this testing program was conducted during periods of minimal or low subscriber usage since it frequently resulted in interference with the information signals being transmitted.
In the case of industrial communication systems utilizing a number of remotely located terminal points to receive and display information, the two-cable system has been generally adopted as a way to provide for the independent transmission of status signals generated at the terminal points. Since these status signals are carried by the second or parallel cable, they do not interfere with transmitted data on the primary cable and are readily received at a monitoring station at the head-end. However, the duplication of transmission lines in a single facility without increasing system capacity when combined with the need to power this second line, substantially increases the capital investment necessary to install such a system. The required investment for a dual cable system is also increased by the need for repeaters in the return path to insure that an intelligible return signal reaches the head-end.
Dual transmission path systems have been preferred to systems using the frequency separation of the information signal transmitted to the subscribers from the monitoring signal at a lower frequency along the same cable since the risk of interference therebetween is eliminated. In the event that a single cable signal is used in the reverse direction for the transmission of monitoring signals, the head-end must deal with the reception of a multiplicity of different signals from non-uniquely addressed terminal points. The identification of the signals as originating from a particular terminal point has proved to be more difficult than anticipated. As a result, these systems have not been widely used in the cable television industry. Furthermore, the use of a single cable for split frequency systems requires the placement of both high pass filters for the transmission of the information signals and a multiplicity of low pass filters to split the band for the return signals. Since these filters are needed at each amplification point in the system, the resultant losses introduced cause the system to need additional amplification, more power and thus create more opportunities for system failure. Thus, the maintenance on the split frequency band communications systems rises significantly.
Accordingly, it is a primary object of the present invention to provide a single cable communications system which permits remotely-located components to be individually addressed and controlled with the responses carried back to the head-end on the single cable without the need for increased amplification.
Another object of the invention is to provide isolation between opposing sides of the transmission line at the site of the remotely-located components being monitored. Further, the system utilizes the existing standby power supplies in a cable system as the sites for repeaters which maintain the signal level of the return monitor signals.
A further object is the provision of a modem/repeater component for reverse transmission in a single cable system which is capable of being retrofit into existing cable systems without requiring substantial redesign thereof.